An Efficient and Selective Oxidation of Sulfides and

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An Efficient and Selective Oxidation of Sulfides and Thiols with Silica-Supported 1,1,3,3-Tetramethylguanidine/Br2 Complex a

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Ahmad Shaabani , Elham Farhangi & Abbas Rahmati

a

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Department of Chemistry, Shahid Beheshti University, Tehran, Iran Published online: 03 Feb 2010.

To cite this article: Ahmad Shaabani , Elham Farhangi & Abbas Rahmati (2010) An Efficient and Selective Oxidation of Sulfides and Thiols with Silica-Supported 1,1,3,3-Tetramethylguanidine/Br2 Complex, Phosphorus, Sulfur, and Silicon and the Related Elements, 185:2, 463-468 To link to this article: http://dx.doi.org/10.1080/10426500902814031

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Phosphorus, Sulfur, and Silicon, 185:463–468, 2010 C Taylor & Francis Group, LLC Copyright ISSN: 1042-6507 print / 1563-5325 online DOI: 10.1080/10426500902814031

AN EFFICIENT AND SELECTIVE OXIDATION OF SULFIDES AND THIOLS WITH SILICA-SUPPORTED 1,1,3,3-TETRAMETHYLGUANIDINE/Br2 COMPLEX

Downloaded by [Texas A&M University Libraries] at 13:02 27 August 2013

Ahmad Shaabani, Elham Farhangi, and Abbas Rahmati Department of Chemistry, Shahid Beheshti University, Tehran, Iran Silica-supported 1,1,3,3-tetramethylguanidine/Br2 complex is an efficient reagent for the selective oxidation of aliphatic and aromatic sulfides to the corresponding sulfoxides and the oxidative coupling of thiols to disulfides in aqueous solution at room temperature in a short reaction time. Keywords Oxidation; silica-supported 1,1,3,3-tetramethylguanidine/Br2 complex; sulfide; thiol

INTRODUCTION Selective oxidative coupling of thiols to disulfides and oxidation of sulfides to sulfoxides are of interest from both biological and synthetic chemistry points of view.1–10 Although a large number of oxidizing agents can affect the conversion of sulfides to sulfoxides and the oxidative coupling of thiols to disulfides,11–24 the susceptibility of sulfoxides and disulfides to further oxidation narrows the choice of reagents for these processes. Therefore, the development of new oxidants for the selective transformation of sulfides to sulfoxides and thiols to disulfides is of importance in synthetic organic chemistry. The use of molecular bromine for the oxidation of sulfides and thiols is restricted because the formation of HBr (as a reduction product) can affect the selectivity of the reaction. Under acidic conditions, the main product becomes contaminated by the formation of side products such as sulfonic acids, sulfinic acids, and bromo-substituted sulfides and sulfoxides.25 Therefore, if bromine is to be used as an oxidant, it is necessary that these problems be circumvented by carrying out the reactions under conditions where HBr is not released as a free acid. A number of amine-based complexes of bromine have been reported in the literature,26 in which amine acts as a HBr acceptor. However, as indicated in Scheme 1, the amine part of the complex is only able to neutralize 1 mol of HBr being produced in the oxidation process. Therefore the reaction media is still acidic from the remaining 1 mol of HBr. Received 16 December 2008; accepted 11 February 2009. Financial assistance from the Research Council of Shahid Beheshti University of Iran is gratefully acknowledged. Address correspondence to Ahmad Shaabani, Department of Chemistry, Shahid Beheshti University, P. O. Box 19396-4716, Tehran, Iran. E-mail: [email protected] 463

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H2O

R2S

R2S=O+2HBr+amine

Amine/Br2 R-S-S-R+2HBr+amine

2RSH Scheme 1


RESULTS AND DISCUSSION This article reports the oxidation of sulfides and thiols to the corresponding sulfoxides and disulfides utilizing 1,1,3,3-tetramethylguanidine/Br2 on hydrated silica gel (Scheme 2).

O

SSTMG/Br2

S 1

R

2

R

CH3CN/H2O 10-40 min

S R1

R2

90-96%

R1=R2= Alkyl or Aryl R1= Aryl, R2= Alkyl

R1SH

SSTMG/Br2 CH3CN/H2O 3-6 min

R1SSR1 90-95%

Scheme 2

This silica-supported 1,1,3,3-tetramethylguanidine/Br2 (SSTMG/Br2 ) oxidant is not only safe to handle owing to full chemisorption of the toxic bromine complex, but both the 1,1,3,3-tetramethylguanidine and silica gel act as a HBr scavenger and prevent the solution from becoming acidic (Scheme 3). The silica-supported 1,1,3,3-tetramethylguanidine/Br2 complex was readily prepared by adding bromine to a magnetically stirred slurried mixture of silica in an n-hexane solution of 1,1,3,3-tetramethylguanidine. The mixture was filtered, washed with n-hexane, and dried at room temperature. This yellow-orange, non-hygroscopic, homogenous solid is very stable at room temperature and is not affected by ordinary exposure to light, air, or water and has none of the offensive odor of bromine or amine. Easy workup and the stability of the reagent make it a safe and convenient source of active bromine. This method offers a simple, general, selective, and highly efficient route for converting sulfides and thiols to their corresponding sulfoxides and disulfides without overoxidation. As shown in Table I, the reaction time for oxidation is relatively short, and the generality of the method was examined using alkyl, aryl, dialkyl, diaryl, and cyclic sulfides as well as alkyl and aryl thiols. It was discovered that a wide variety of sulfides and thiols can be selectively oxidized by this inexpensive reagent under mild reaction conditions.

OXIDATION OF SULFIDES AND THIOLS

465

X

N

H

O H

Si OH R

N

Br

Br

N

H O

H Si

R XNH2Br

O H

O S

Br


R2S

H

H

X N Br

S R

R

X=N +

Br S R OH2 Br R

Br X N

H Br

S R

R

H2O Br

Scheme 3

The possible mechanism for the oxidation of sulfides to the corresponding sulfoxides using silica-supported 1,1,3,3-tetramethylguanidine/Br2 complex is outlined in Scheme 3. In conclusion, we report an easy and versatile method for the selective oxidation of sulfides and thiols to their corresponding sulfoxides and disulfides without any over-oxidation. The amine and silica parts of the complex act as HBr acceptors, preventing the solution from becoming acidic. The formation of 1,1,3,3-tetramethylguanidine/hydrobromide as the reaction proceeds seems to accelerate the rate of oxidation possibly by increasing the ionic strength of the solution.

EXPERIMENTAL Melting points were measured on an Electrothermal 9200 apparatus. IR spectra were recorded on a Shimadzu IR-470 spectrometer. 1H NMR spectra were obtained on solutions in CDCl3 and DMSO-d6 . All reaction products were known and characterized by IR and 1H NMR spectra and melting points as compared with those obtained from authentic samples. All chemical reagents were purchased from Fluka and Merck and were used without purification.

Preparation of Silica-Supported 1,1,3,3-Tetramethylguanidine/Br2 A solution of bromine (3.84 g, 24 mmol) in n-hexane (20 mL) was added to a magnetically stirred slurry mixture of silica gel (16.00 g) in n-hexane (100 mL) solution of 1,1,3,3-tetramethylguanidine (2.76 g, 24 mmol). The mixture was stirred for an additional 1 h, and then the product was collected by vacuum filtration as yellow solid; yield 20.88 g.

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Table I Oxidation of sulfides and thiols with the silica-supported 1,1,3,3-tetramethylguanidine/Br2 complex

Entry

Reductant

Yield (%)

M p or Bp (◦ C) Found (Reported) [ref.]

(CH3 (CH2 )3 )2 S

(CH3 (CH2 )3 )2 SO

15 10

94 92

27–29 (29–31.6)a [27] 232–234 (235–237)b [27]

3 4 5 6 7 8 9 10

PhSCH3 p-BrC6 H4 SCH3 p-O2 NC6 H4 S CH3 PhSCH2 CH3 PhCH2 SCH2 Ph PhCH2 SPh PhSPh

PhSOCH3 p-BrC6 H4 SOCH3 p-O2 NC6 H4 SOCH3 PhSOCH2 CH3 PhCH2 SOCH2 Ph PhCH2 SOPh PhSOPh

30 15 20 10 10 25 20 40

96 90 91 95 93 92 94 93

31–33 (33–34)a [28] 73–75 (74–76)a [28] 139–141(140–142)a [28] 141–144 (146)a [28] 129–132 (133–135)a [28] 120–122 (123–124)a [28] 68–70 (70.5)a [29] 150–153 (53–155)a [30]

11 12 13 14 15 16 17 18 19

CH3 (CH2 )2 SH CH3 (CH2 )3 SH CH3 (CH2 )7 SH C6 H11 SH PhCH2 SH PhSH p-CH3 C6 H4 SH o-ClC6 H4 SH

(CH3 (CH2 )2 S)2 (CH3 (CH2 )3 S)2 (CH3 (CH2 )7 S)2 (C6 H11 S)2 (PhCH2 S)2 (PhS)2 (p-CH3 C6 H4 S)2 (o-ClC6 H4 S)2

3 4 5 5 3 6 5 5 3

90 92 91 94 95 93 95 92 95

193–194 (193)b [31] 190–191 (192)b [31] 72–75 (74–75)a [28] 120–123 (125–130)a [29] 67–71 (69–72)a [29] 57–59 (61–62)a [29] 44–46 (45–46)a [32] 84–86 (87–88)a [30] 135–137 (139)a [33]

1 2


Product

Time (min)

aMelting bBoiling

point. point.

Oxidation of Sulfides In a typical reaction, methyl phenyl sulfide (0.124 g, 1 mmol) was added to the mixture of silica-supported 1,1,3,3-tetramethylguanidine/Br2 (1 g) in aqueous acetonitrile (CH3 CN:H2 O; 3:1 v/v, 20 mL). The mixture was stirred at room temperature for 30 min, while the progress of the reaction was followed by TLC (n-hexane:ethylacetate 7:1). The product was extracted into CH2 Cl2 (2 × 10 mL) and dried over anhydrous magnesium sulfate. Evaporation of the solvent gave a product (0.134 g, 96%) of sufficient purity for most purposes; Mp 31–33◦ C (ref.28: 33–34◦ C).

Oxidation of Thiols In a typical procedure, p-methyl thiophenol (0.124 g, 1 mmol) was added to a mixture of silica-supported 1,1,3,3-tetramethylguanidine/Br2 (0.5 g) in aqueous acetonitrile (CH3 CN:H2 O; 3:1 v/v, 20 mL). The mixture was stirred at room temperature for 3 min, while the progress of the reaction was followed by TLC (n-hexane:ethylacetate 7:1). The

OXIDATION OF SULFIDES AND THIOLS

467

product was extracted into CH2 Cl2 (2 × 10 mL) and dried over anhydrous magnesium sulfate. Evaporation of the solvent gave a product (0.118 g, 96%) of sufficient purity for most purposes; Mp 44–46◦ C (ref.32: 45–46◦ C).


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